The Obama administration took a concrete step toward curbing the flow of semiautomatic weapons to Mexico last week when it adopted a new regulation mandating the reporting of multiple sales of long guns to federal authorities.

Under the regulation, some 8,500 licensed gun shops in Arizona, California, New Mexico and Texas will be required to inform the Bureau of Alcohol, Tobacco, Firearms and Explosives when a customer buys more than one semiautomatic that is .22 caliber or greater within a five-day period.

The regulation is a small but significant tool that could help federal authorities keep weapons sold in the United States out of the hands of Mexican gangs and drug cartels. Rather than tracing an AK-47 after it has been recovered from a crime scene, ATF agents may be able to intervene before the weapon is smuggled across the border.

The National Rifle Assn. is, not surprisingly, denouncing the modest rule as encroaching on Americans' 2nd Amendment rights; in fact, it is already threatening to sue the federal government, contending that only Congress can impose such rules. And some Republicans have noted that the decision to impose what they consider unnecessary reporting standards on Americans comes in the wake of a bungled ATF operation called Fast and Furious. That operation was designed to track illegal straw purchases of guns destined for Mexico, but the agency lost track of the guns and some of them ended up at crime scenes, including at the site of the killing of a U.S. Border Patrol agent last year.

The ATF should be required to explain how Fast and Furious was botched. But that failure shouldn't be used to derail a perfectly sensible plan that is, if anything, too modest. The new rule doesn't impinge on anyone's right to own a gun. It doesn't even prohibit individuals from making multiple purchases. It simply requires that licensed gun dealers report such sales. Dealers are already subject to the same reporting requirement for multiple handgun sales, and in states such as California, a customer can buy only one handgun a month.

Military-style weapons are fueling the violence in Mexico, where nearly 40,000 people have died in the last four years. The new rule won't stop the bloodletting. It will, however, provide agents in this country with tips that could help rein in the illegal movement of weapons and save lives on both sides of the border.

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